The present invention relates to improvements in apparatus for treating flowable solid materials, such as building materials, fertilizers, mixtures of two or more flowable solid ingredients and the like. More particularly, the invention relates to improvements in apparatus for mixing and metering flowable solid materials.
German Offenlegungsschrift No. 23 40 246 discloses an apparatus wherein the outlet in the lower portion of a silo for a supply of stored flowable solid material is disposed at a level above a mixing unit embodying a continuously operated mixing pump whose housing is suspended on and below the outlet of the silo. Such mounting of the mixing pump on the silo is intended to obviate the need for a pneumatic conveyor which operates with compressed air and, in certain conventional apparatus, is used to transport flowable material from the main source to the mixing pump. The directly mounted mixing pump, its motor, the shaft which is driven by the motor and a second pump which evacuates the intermixed materials are mounted on the silo in an inclined position so that the material which is being treated advances downwardly and laterally on its way from the outlet of the normally upright silo or an analogous source of untreated material. Such inclined mounting of the mixing unit necessitates the provision of an intermediate piece or adapter which connects the region of the outlet of the silo with the inlet of the mixing pump and acts not unlike a flow restrictor. In other words, the material cannot issue from the silo at the rate which would be possible in view of the cross-sectional area of the outlet. Moreover, inclined mounting of the mixing unit below the outlet of an upright silo or an analogous source of flowable material is undesirable when the material in the silo contains several ingredients which are likely to become separated as a result of a more or less pronounced classifying action which takes place during flow of intermixed ingredients through the aforementioned adapter and into the inlet of the mixing pump. Such tendency of intermixed ingredients to become separated from one another depends on their specific weight, their particle size and/or other parameters such as the length of the path between the outlet of the silo and the inlet of the mixing pump, the slope of the path which is defined by the adapter, the extent of the flow restricting action of the adapter and others. Still another drawback of the just discussed conventional mixing apparatus is that, owing to inclined mounting of the mixing pump, mixing shaft, evacuating pump and other components of such apparatus, the wear upon the bearings for the rotary parts is not uniform. The same holds true for the blades, paddles, arms or analogous mixing elements which are used in such units.
German Pat. No. 29 03 373 discloses a modified mixing apparatus which is mounted in a substantially horizontal position or slopes slightly upwardly for the purpose of admitting treated material into receptacles having relatively high sidewalls. Such mixing units are quite satisfactory for the admission of treated materials into containers whose rims are disposed at or above the level of the mixing unit. A drawback is that the direction of the material flow must be reversed or otherwise drastically changed on the way from the outlet of the source to the container or containers for the treated material. Thus, the material issues from the source by flowing vertically downwardly, and the material thereupon flows horizontally or upwardly in order to enter a receptacle. Such changes in the direction of flow of treated material are highly undesirable under a number of circumstances, for example, when the material is not readily flowable or when the material is a mixture of several ingredients which tend to become separated from one another. The tendency of various ingredients to become separated from each other is especially pronounced if the ingredients have different particle sizes, densities and/or shapes as well as if the ingredients tend to densify. Such types of materials cannot be metered with a requisite degree of accuracy because the predictability of metering action is adversely influenced by changes in the extent to which the various ingredients are intermixed with one another. Consequently, if the mixing operation involves the admission of binding agents and/or other types of additives, the additives must be admitted in larger quantities than absolutely necessary in order to avoid the formation of batches wherein the percentage of additives is too low. This contributes to the cost of the treated product, especially if the additives are expensive binding agents or the like. For example, when the material which is discharged by way of the outlet in the lower part of a silo is a building material which must be mixed with cement or the like, a minimum percentage of cement must be present in each batch of the treated product in order to ensure that the finished product will exhibit a certain minimum amount of stability after setting.
Another conventional mixing apparatus for building materials or the like is disclosed in German Offenlegungsschrift No. 20 34 837.